Population

Population

North Atlantic. The International Whaling Commission (IWC) recognizes three stock divisions for sei whales in the North Atlantic: Nova Scotia; Iceland-Denmark Strait; and Eastern (including the waters of Spain, Portugal, British Isles, Faeroes and Norway), but the divisions were chosen on largely arbitrary grounds (Donovan 1991).

About 14,000 sei whales are recorded caught by modern whaling in the North Atlantic. In addition, an unknown proportion of the approximately 30,000 unspecified large whales caught in the North Atlantic in the late 19th and early 20th centuries were sei whales.

Sei whales appear to have been depleted in the eastern North Atlantic, with over 7,500 recorded taken in Norwegian and British waters prior to World War II (not counting the unspecified whales), but fewer than 200 since then. The species seems to be virtually absent there now: Norwegian surveys of northeast Atlantic waters in 1987, 1989, and annually from 1995–2005 have yielded only one sei whale sighting in the Eastern stock area. Small numbers of sei whales were caught during 1957–80 off northwestern Spain, mainly in late summer/autumn (Aguilar and Sanpera 1982); the stock identity of these animals is unclear.

By contrast, sei whales are still abundant in the central North Atlantic (Iceland-Denmark Strait IWC stock area), especially southwest of Iceland south to 50°N in summer. The only survey with reasonably complete coverage, designed specifically for the purpose of estimating sei whale abundance, was in 1989, and yielded a population estimate of 10,300 whales (CV 0.27) (Cattanach et al. 1993). Areas of sei whale abundance seem to shift markedly between years relative to the northern extent of the distribution, more so than for other baleen whale species (Gunnlaugsson et al. 2004).

No recent abundance estimates are available for the western North Atlantic. The population size during 1966–69 was estimated at 2,078 (Mitchell and Chapman 1977) from sightings surveys, using strip transect methodology. About 1,200 sei whales were taken off Nova Scotia during 1962–72.

North Pacific. The last assessment of North Pacific sei whales by the IWC Scientific Committee was in 1974 (IWC 1977). The exploitable population (animals of legally catchable size) was estimated to have declined from 42,000 in 1963 to 8,600 in 1974 (Tillman 1977). Over 40,000 sei whales were caught during this period (IWC 2006). A 75% reduction in sei whale catch rates of Californian whaling stations during the 1960s is consistent with an ocean-wide decline (Rice 1977). Exploitation ceased in 1975. The extent to which the population has recovered since then is unclear.

Hakamada et al. (2004) gave an estimate of 4,100 animals from one area of the western North Pacific, but attempts to extrapolate this to produce an estimate for the entire western North Pacific have not been accepted. It is likely that there has been some increase in the population since the end of exploitation, but given the tendency for sei whale distribution to change from year to year, it is hard to interpret the limited survey data that currently exist.

Sei whales appear not to have formerly been uncommon in the eastern North Pacific. However, a preliminary estimate for the US west coast based on surveys in 1996 and 2001 is only 56 whales (Barlow 2003). In the Eastern Tropical Pacific, all potential sei/Bryde’s whales that could be specifically identified were found to be Bryde’s, suggesting that sei whales are now rare (Wade and Gerrodette 1993).

Southern Hemisphere. Over 200,000 sei whales are recorded taken by modern whaling in the southern hemisphere during 1905–1979. The IWC arbitrarily divides southern hemisphere sei whales, along with other southern hemisphere baleen whales, into six longitudinally defined management Areas. Sei whales in all of the six Areas have been classified as Protection Stocks by the IWC since 1978. The greatest catches were during 1960–72, when they exceeded 5,000 in every year, topping at nearly 20,000 in 1964 alone. Most catches were taken in summer by pelagic fleets operating south of 40°S, but winter catches were also taken from land stations in Brazil, Peru and South Africa (and smaller numbers in Chile, where there is ambiguity with Bryde’s whales).

The last assessment of southern hemisphere sei whales by the IWC Scientific Committee was conducted in 1979 (IWC 1980). Some extra information on these assessments is given by Horwood (1987). Based on catches/sightings per unit effort from Japanese catch and scouting vessels respectively, the “exploitable” stock size (sei whales of legal size, approximately 2/3 of total population), excluding Area II (South Atlantic sector), was estimated to have declined from about 64,000 in 1960 to about 11,000 by 1979. The population model used failed to produce an estimate for Area II, because nearly all the catch for this area was taken in just the two seasons 1964/65 and 1965/66, with nearly 30,000 sei whales being taken in these two years, yet the abundance indices suggested a more continuous decline during the 1960s and 1970s (IWC 1980). This suggests that the management Areas do not correspond to biological populations.

Other available evidence also suggests a severe and rapid decline in sei whale stocks during the 1960s and 1970s. The catch and sighting rates from winter whaling operations off Brazil and South Africa show even sharper declines. Abundance indices from the Brazilian whaling ground for sei whales during 1966–81 (da Rocha 1983) declined by ca. 90% during 1966–72.

No recovery of sei whales in Brazilian waters has been detected since that time (Zerbini et al. 1997, Andriolo et al. 2001). Catch and sightings indices off Durban, South Africa for 1965–74 (Best 1976) declined by over 95%. Tag-recapture data from pelagic whaling suggested an approximately 6-fold decline between 1962–76 (IWC 1980, Fig. 1).

IWC (1996, Table 2) gives a total population estimate for sei whales south of 30°S of about 10,000 based on a combination of IDCR and Japanese Scout Vessel (JSV) sightings data. It is difficult to know how much reliance to place on this estimate, because no variance is given, but it is consistent with a severe decline. No more recent summer data are available, except for the area south of 60°S, which is outside the main summer range of sei whales. In the absence of dedicated surveys in sei whale habitat, and resulting abundance estimates, it is not possible to assess whether there has been any increase in southern hemisphere sei whales since the cessation of whaling.

Generation time and maximum rate of increase. The generation time is estimated to be 23.3 years (Taylor et al. 2007). The 3-generation time window for applying the decline criterion (A) is 1937–2007.

For assessment purposes, the IWC Scientific Committee has used a natural mortality rate of 0.06, an age at first reproduction declining (with stock depletion) from 12-13 years to 10-11 years, and an annual pregnancy rate increasing (with stock depletion) from 0.27 to 0.37-0.39 (IWC 1980; Horwood 1980). Assuming a maximum annual pregnancy rate of 0.40, a minimum age of first reproduction of 10 years, and no juvenile mortality, the maximum rate of increase is 2.7% p.a. Horwood and Millward (1987) also conclude that the maximum rate of increase for sei whale populations is less than 3% per year.

Population assessment. Because the available published assessments for this species are not up to date, an updated population assessment is conducted here to enable assessment of the population reduction over the period 1937-2007 relative to the A criterion. While the available data do not permit a scientifically rigorous estimation of the extent of population reduction, it is reasonable to use conventional population assessment methods to provide a crude indication of the extent of possible reduction relative to the criteria. A conventional deterministic age-structured model with an age at first capture (“recruitment”) (ar) and an age at first reproduction (am), and linear density-dependence was applied to the North Pacific, North Atlantic and Southern Hemisphere regions separately. The results are provided in the linked PDF document, and constitutes an integral part of this assessment.